1. Field of the Invention
This invention relates to an antenna disposed on a windowpane of a motor vehicle having an electrically conductive motor vehicle body. The windowpane has a substantially rectangular or trapezoidal heating field that is provided on each side with a bus-bar and has bus-bar connections for feeding heating current on both sides. A heating direct current source is connected to the heating field and is electrically connected to the electrically conductive body of the motor vehicle. The current is fed on each side via an inductively high-resistance current feed network which is installed within proximity of the side edges of the windshield. The heating field is largely high-frequency insulated against the body of the motor vehicle with the help of current feed networks due to their high impedance so that the heating field can conduct high-frequency voltage, that is insulated from the body of the motor vehicle.
2. Description of the Prior Art
A heating field inductively connected in such a way can thus be designed as an antenna with the help of the current feed networks, as shown, for example in FIG. 1 of German Patent DE 36 18 452. The high-frequency coupling to a heating field conducting the high-frequency voltage, to form the antenna, can be accomplished, for example, by a connection to a bus-bar of the heating field.
It is found in automobile construction that interference signals frequently caused by the electronic noise of the automobile are coupled in via the longer current feed lines connected to the bus-bars without any HF-effective filter means. These interference signals disturb the reception in undesirable ways. The advantage offered by the current feed networks installed on the two sides near the bus-bar lies in the possibility of a high-frequency connection of the heating current feeds to the auto body on each side of the respective feed network, facing away from the heating field, without requiring the current to be conducted by longer lines on both sides of the heating field.
Furthermore, high-frequency impedance conditions can be defined on the bus bars. These conditions are not dependent upon the way in which the heating current lines are configured. However, the problems connected with this arrangement are in providing a inductance value for heating currents, with intensities of up to 30 A, particularly within the range of AM radio transmission. The required inductance cannot be realized in the conventional way with small antennas and with light-weight feed networks. The invention is based upon designing feed networks of high inductance that are constructed as small as possible. Also, at low frequencies, the feed networks should have efficient RF insulation, and have adequately low high-frequency losses and filament wattage losses.